Pressurized Thermal Shock (PTS) is a safety issue receiving much attention by the NRC and other organizations in the nuclear industry. The problem is the loss of ductility in ferritic steel pressure vessels due to neutron irradiation over vessel life. The scenario of concern is a rapid cooldown due to an accident, possibly causing a crack in the pressure vessel due to thermal stresses induced by cold water coming in contact with the vessel wall. Subsequent repressurization of the vessel aggrevates the thermal-induced crack.
Within the pressure vessel's construction of particular concern are the weld areas which are most likely to suffer loss of ductility due to irradiation. Although a number of possible methods are being considered to prevent and/or mitigate PTS, vessel annealing is the only method of restoring ductility, thereby increasing PTS resistance for high embrittled vessels.
The annealing process involves heating the pressure vessel, and/or the area of concern in the vessel, up to 750.degree. F.-800.degree. and maintaining that temperature over a five-day period. A number of methods have been considered for annealing, such as: resistance heating, combustion heating, internal heated liquid, and external heated liquid. All these methods of vessel annealing have positive and negative aspects. One significant negative attribute in common with all these methods of annealing is that substantial radial temperature gradients will be induced across the vessel wall, despite the fairly good thermal conductivity of carbon steel (K-30 BTU/hr ft.sup.2 .degree. F.).
A source of heat is needed with which to bring the pressure vessel up to the temperature required for annealing without the drawback described above.